1. Field of the Invention
The present invention relates to photothermographic material and a method of forming an image using the photothermographic material.
2. Description of the Related Art
Recently, in the field of films for medical imaging, there is a strong demand for reducing the volume of waste processing liquid from the viewpoint of environmental preservation and economy of space. There have been demands for technologies relating to use of a photothermographic material as a film for medical imaging and a film for graphic arts. In particular, there is a demand for a photothermographic material that can be efficiently exposed by a laser image setter or a laser imager, and can provide black-toned images with high resolution and sharpness. Such a photothermographic material can provide users with a more simple and ecological thermal developing system without the use of liquid processing chemicals.
Although there are similar demands in the field of general image forming materials, high image quality (i.e., excellent sharpness and fine graininess) is particularly required for images used in medical imaging where high image quality of excellent sharpness and granularity are necessary. Further, images with blue-black tones are preferred from the perspective of facilitating diagnosis. Various types of hard copy systems using pigment or dye, such as an inkjet printer and an electrophotograph system, are commonly used as a general image forming system. However none of these is satisfactory as an output system for medical images.
In general, thermal image forming systems using organic silver salts are described in U.S. Pat. Nos. 3,152,904 and 3,457,075 and on pages 279 to 291, Chapter 9, “Thermally Processed Silver Systems,” written by D. Klosterboer, in (Imaging Processes and Materials) Neblette, 8th edition, compiled by J. Sturge, V. Walworth and A. Shepp (1989), the disclosures of which is incorporated herein by reference.
A photothermographic material typically includes a photosensitive layer in which a photocatalyst (e.g., silver halide) of a catalytically active amount, a reducing agent, reducible silver salt (e.g., organic silver salt) and a toner for controlling the tone of a developed silver image as needed are dispersed in the matrix of a binder. After an image is exposed thereon, the photothermographic material is heated to a high temperature (e.g., 80° C. or above) to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (which acts as an oxidizing agent) and a reducing agent, thus providing a black silver image. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image of the exposed silver halide. As a result the black silver image is formed in the exposed region (see U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No. 43-4924). Further, Fuji medical dry imager FM-DP L is an example of a practical medical image forming system using a photothermographic material that has been marketed.
In production of a thermographic system using an organic silver salt, two methods are available: in one method, a solvent coating is adopted and in the other method a coating liquid containing polymer fine particles as a main binder in an aqueous dispersion is applied and dried. In the latter method, since no necessity arises for a process of solvent recovery or the like, a production facility is simple and the method is advantageous for mass production.
Since such a thermographic system using an organic silver salt has no fixing step, there has been a considerable problem in image preservability after development, particularly with respect to degradation of a print-out when exposed to light. As means for improving the print-out, a method in which silver iodide formed through conversion of an organic silver salt is employed is disclosed in U.S. Pat. No. 6,143,488 and EP No. 0922995. In the method, such as described here, in which an organic silver salt is converted with iodine, however, a sufficient sensitivity cannot be obtained, which has led to difficulty in incorporation into an actual system.
As to other photosensitive materials using silver iodide, description thereof is given in WO (Laid-Open) Nos. 97-48014 and 97-48015; U.S. Pat. No. 6,165,705; Japanese Patent Application Laid-Open (JP-A) No. 8-297345; Japanese Patent No. 2785129 and others, in any of which neither a sufficient sensitivity nor a sufficient fog level is achieved, leading to a poor laser exposure photosensitive material which is not suitable for practical use. Development of a method has been awaited in which a silver halide rich in silver iodide content is effectively used to full performance.
JP-A No. 2000-305213 discloses an image forming method using a blue to ultraviolet laser beam and a photosensitive material therefor. However, no silver iodide is employed, resulting in insufficient sensitivity.
It has been understood from research conducted by the inventors of the invention that a photosensitive material of a high sensitivity and a high image quality is obtained by use of silver iodide in a system using a blue to ultraviolet laser beam.
Though a photosensitive material using silver halide with a high silver iodide content ensures practical sensitivity, the sensitivity thereof is still insufficient as compared with a level of expectation based on exposure being conducted at an absorption edge unique to silver iodide at which a high absorbance is realized.
That is, while it has been desired to achieve a full performance of a silver halide with a high silver iodide content since it is excellent in suppression of fogging and print-out, the current state of the art remains of insufficient sensitivity.